The emergency and service requirements of modern heavy-duty highway tractor-trailer combinations meeting all of the pertinent governmental regulations that are applied to the trucking industry have necessitated installing a plurality of complex semi-trailer brake valve arrangements. The valve arrangements, which have been successfully designed to meet the requirements, are expensive and pose installation and maintenance problems. A system that is typical of the known valve arrangements includes a plurality of valves with multiple complex inter-connections, all designed to carry out multiple requisite brake functions.
One of the functions of the known multiple brake valve systems is a pressure protection valve that maintains pressure in the air supply line between the steering and at least one steerable axle in the event of failure of a pressurized air reservoir of the vehicle. This function is important because it is designed to prevent automatic application of spring brakes that are activated once the pressure in the air supply line drops below a predetermined low pressure.
A further function of the multiple brake systems relates to a check valve arrangement employed on the heavy-duty vehicles to block bleeding of pressurized air from the pressurized air reservoir through an air supply port.
Another function of the brake valve systems is to allow pressurized air from the air reservoir to be applied to the service brakes of the semi-trailers if the main air supply system has failed.
Still another function, which is critical to a modern tandem braking system, is the use of antilock brakes (ABS) during driving and braking operations of the semi-trailers. As known, ABS brakes increase safety by eliminating lockup and minimizing the danger of skidding, thereby allowing the semi-trailers to stop in a straight line. ABS brakes also allow the driver to maintain steering control during heavy braking so the vehicle can be driven to avoid an obstacle or another vehicle. Thus, ABS brakes optimize braking by monitoring the relative speed of the wheels to one another to module brake pressure as needed to control wheel slippage and maintain traction when the brakes are applied.
A tractor-trailer tandem braking system typically employs a multi-port relay valve to control the operation of the service brakes. The relay valve supplies pressurized air to, maintains pressurized air in, and releases pressurized air from the service brake chambers pursuant to control signals that are received from the tractor.
Manifestly, the various valve installations and interconnections accomplishing only partially the above-discussed functions on a multi-axle vehicle are complex, costly, difficult and inconvenient to install, troubleshoot, service, and maintain and otherwise are undesirable from a parts inventory standpoint. Previous efforts to integrate the various valve requirements within a single envelope or housing have not resulted in brake systems that could successfully meet current requirements. Moreover, a single housing enveloping ABS, regular service, and pressure protection valves has not been suggested by the known prior art.